1. Field of the Invention
The present application relates, generally, to silica glass materials, and, more particularly, to modified rare-earth doped silica glass materials for use in optical fiber amplifiers, ASE sources and lasers.
2. Description of the Related Art
The extraordinary advancement of wide area networking services, e.g., the Internet, over the past several years has been enabled by the confluence of two key technologies, i.e., the erbium doped optical fiber amplifier, EDFA, and wavelength division multiplexing, WDM. Since the discovery by Townsend and Payne in the late 1980""s of a method for fabricating high quality rare-earth doped silica fibers, much work has centered on the development of and the exploitation of the EDFA. The typical EDFA consists of Er3+ doped into an alumino-silicate glass optical fiber. The developments have revolutionized the telecommunications industry as EDFA has replaced electronic repeaters in fiber based networks. The EDFA coupled with the development of WDM technology has allowed for the engineering of high bandwidth optical systems in the region of 1525 to 1570 nm. This is within the xe2x80x9clow-lossxe2x80x9d or xe2x80x9cthirdxe2x80x9d optical fiber telecommunications window. The low-loss window is the range 1420 nm to 1650 nm where the attenuation per unit length for silica optical fiber is near its minimum, e.g.,  less than 0.35 dB/km. The C-band 1525 to 1585 nm, and L-band, 1585 to 1650 nm, are each covered by the EDFA, but it is apparent that these two bands represent a portion of the low-loss region for silica but not the total. Due to the fortunate coincidence of the Er3+ gain transition with the low-loss window, the EDFA has come to be extensively used in optical fiber telecommunications systems. The EDFA has also enabled the transmission of enormous quantities of data via WDM, that is, by providing gain simultaneously for multiple data transmission channels at different wavelengths within the bandwidth of the EDFA. To date no practical amplifier has been demonstrated for wavelengths of  less than 1520 nm, so that fully half of the low-loss window bandwidth is unused.
There is a desire for the development of the S-band amplifier. This requires that the rare-earth ion with an appropriate transition have fluorescence in the region of approximately 1450 to 1520 nm. Tm3+ has the necessary fluorescence. The relevant transition is 3H4 to 3F4, which fluoresces at 1430-1500 nm. In the absence of nonradiative quenching, the lifetime of the upper level, 3H4, is expected to be approximately, 1.5 ms; this is observed for Tm3+ in low phonon energy fluorozirconate glasses. However, the energy separation between 3H4 level and the next lower level, 3H5, is sufficiently small, 4400 cmxe2x88x921, that the upper level is substantially quenched by multiphonon processes in high-phonon energy glasses like the silicates. The lifetime has been measured as  less than 20 xcexcs in a pure silica host. Depletion of the upper state lifetime via nonradiative processes reduces the population available to provide gain on the transition of interest. While fiber amplifiers based on this transition have been demonstrated in fluorozirconate glasses, these have proved impractical due to various problems with the host material.
Thulium, Tm, has a 3H4 to 3F4 transition which provided amplification in the S-band wavelength range using a fluorozirconate host. This fluorozirconate material possesses properties that do not lend the material for use in lasers or in optical fibers. These materials are hygroscopic, prone to formation of micro-crystallites over time and have glass transition temperatures at about 400xc2x0 C. which prevents fusion splicing to standard telecommunications-grade fibers. In the event these glasses are butt spliced they tend to become damaged with heavy pumping.
Although the fluoride and tellurite hosts doped with thulium offer high quantum efficiencies for the 1.47 xcexcm transition, some of the material""s properties are problematic with respect to making a practical device. Fluoride glasses are very difficult to fabricate into low-loss fiber due to a propensity towards crystallization and suffer from poor chemical durability. Tellurite glasses, although stable, have a high index of refraction and high thermal expansion, which complicates splicing into an all-optical system.
With the advent of new silica fibers with low-loss across the entire region of 1200 to 1600 nm, i.e.,  less than 0.35 dB/km, optical amplifiers that can potentially amplify at other wavelengths within this region are increased importance.
Silica host materials do have both good chemical and mechanical properties, e.g., fusion splicing to the silicates, high mechanical strength, high glass transition temperature, and extremely low thermal expansion. However, doping silica materials with Tm3+ has low fluorescence and high phonon quenching and therefore not practical for use in optical fiber systems.
Accordingly, it is an object of the present invention to provide a silica glass material doped with Tm3+, Ho3+, and Tm3+-sensitized-Ho3+ in which the material has reduction in the multiphonon quenching compared to the multiphonon quenching of pure silicates.
It is a further object of the present invention to maximize the lifetime of the radiating ions in the 3H4 level for thulium in silica.
It is a further object of the present invention to prepare a glass material that can be fusion spliced directly to conventional silica fibers.
It is a further object of the present invention to prepare a doped glass material that can be used as an amplifier.
It is a further object of the present invention is to improve the efficiency of fluorescence for a doped silica glass.
It is still a further object of the present invention to increase the fluorescence quantum efficiencies for the 1.47 xcexcm transition for a Tm3+ doped silica glass material.
It is yet a further object of the invention is to provide a silica glass composition that provides fluorescence in the S-band region of approximately 1450 to 1520 nm.
It is a further object of this invention to provide silica glass dopant with holmium and thulium sensitized holmium that exhibits improved radiative efficiency.
It is a further object of this invention to use the modified silica glass composition as a laser, an amplifier and ASE source.
These and additional objects of the invention are accomplished by the structures and processes hereinafter described.
The present invention relates to a modified silica glass providing a reduction in the multiphonon quenching for a rare-earth dopant that contains: SiO2 in a host material; a rare-earth oxide dopant selected from the group consisting of Tm3+, Ho3+ and Tm3+ sensitizedxe2x80x94Ho3+; a first SiO2 modifier; in which the first modifier is a 3+ cation dopant, and the first modifier is selected from the group consisting of Ga, Y and combinations thereof such that the first modifier reduces multiphonon quenching of the rare-earth dopant contained therein.
The present invention in another aspect of the composition is made by made by combining: between about 70 and about 99 molar percent SiO2 in a host material; between about 100 and about 100,000 ppm by weight of a rare-earth oxide dopant selected from the group consisting of Thulium, Holmium and Thulium-sensitized-Holmium; between about 0.1 and about 20 molar percent of a first modifier; and between about 0.1 and about 10 molar percent of a second modifier; such that the first and second modifiers reduce multiphonon quenching of the rare-earth contained therein.
The present invention is another aspect a silica glass composition containing rare-earth dopant and at least one modifier selected from the group consisting of Ga, Y and combinations thereof which is suitable to reduce multiphonon quenching for the rare-earth dopant so that the rare-earth dopant permits significant emission at a wavelength between about 1.4 to about 2.0 xcexcm when pumped.